Jonathan Silverlight wrote:
In message , Andrew Yee
writes
McDonald Observatory
University of Texas

Contact:
Rebecca A. Johnson
ph: 512-475-6763 fax: 512-471-5060


9 December 2003

Astronomers Re-measure the Universe with Hubble Space Telescope

One of the lower rungs is knowledge of the distance to the Large
Magellenic Cloud (LMC) -- one of the satellite galaxies of the Milky
Way. Astronomers' knowledge of the LMC's distance is based in large
measure on Cepheids inside that galaxy. The problem is, those Cepheids
are not made up of the same stuff as the ones in our galaxy. So
astronomers aren't sure if the P-L relationship really works right on them.

If Cepheids in the LMC aren't the same as the ones in our galaxy, is
there any reason to think that Cepheids in galaxies in the Virgo cluster
are ???

The concern is that Cepheid properties might vary somewhat with stellar
metal abundance, wtih the LMC being down by ~3x with respect to our
region of the Milky Way disk. In this property, various Cepheids in
Virgo would lie mostly between solar and LMC abundances, since many are
in the outer disks of luminous spirals. Cepheids are massive, hence
short-lived, and their chemistry will reflect the current state of the
interstellar medium. Since many spirals have gradients in ISM metal
abundance (decreasing outward), the HST data themselves have been able
to put limits on such changes, since it sees the same period-luminosity
relation for radial subsets of Cepheids in several galaxies.

Still, it's always really really good to check important conclusions
in as many ways as one can.

Bill Keel